Adventurers and Exiles: The Great Scottish Exodus by Marjory Harper 422pp, Profile, £25

The story of the exodus from the Scottish Highlands is commonly told in three stages: eviction, destitution and emigration. The Highland clearances, during which thousands of peasants were forced out of their villages, which were then destroyed and the land given over to "sheep walks", are as much a feature of mythology as of history. The mass emigrations, beginning in earnest in the late 18th century and lasting through most of the 19th, are Scotland's diaspora and stand as a defining event in the shaping of the modern nation. Marjory Harper calls frequently on tales of callous evictions, but she has another story to tell: emigration, particularly from the Highlands but also from the Lowlands and the industrial cities, was often voluntary, propelled by a desire to leave "old Scotland" and make a better life in the new world.

Adventurers and Exiles is an academic book camouflaged as a mainstream commercial one - the author is a senior lecturer in history at Aberdeen - but it has enough human interest and narrative force for the disguise to work. Harper draws on a remarkable fund of sources: letters, diaries kept on voyages, orphanage archives and oral testimony, much of it previously unpublished. Canada was the main attraction for emigrants, with Australia and New Zealand next (the US has never been as popular with Scots as with the Irish).

The mythology states that as much as the prosperous settlers of Quebec and New South Wales were keen to find people to work their farms, the landowners of Sutherland and Ross-shire wanted them out. But again Harper contradicts the notion that forced evictions were universal; many Scottish landlords lamented the draining of their workforces. Whether you left by choice or not, the ocean voyage to reach your destination was a dreadful prospect. Thirty days at sea was average to reach North America, twice as long to get to Australia.

The more genteel passengers were put out by the habits of some Highlanders, who were described by one Canadian official as being "like the Indians in that they spend a great deal of time in talking over their grievances, real or fancied". Some used their dinner plates as chamber pots at night, and were not fond of bathing or fresh air. Passengers in cabin class avoided much of the distress but, as Harper says, "seasickness was no respecter of persons".

Among the most affecting documents in the book is a diary kept by Charles Robertson, an articulate and sensitive 13-year-old. He sailed to Quebec in 1846, along with his parents and six siblings. His mother was on the point of giving birth:

The orphanages of William Quarrier and Thomas Barnardo were large suppliers of destitute children to the colonies, where they might enjoy "the reward of hard work". Some stories, such as that of 14-year-old Annie who was worked to the bone, beaten by her mother and forced to sleep on "two pieces of wet and filthy sacking", are reminiscent of recently publicised cases. Despairing parents would bring uncontrollable children to Quarrier, with the recommendation: "Canada's the right place for him." But many Quarrier children wrote letters home full of gratitude for the opportunities the move had given them. "Quarrier was a firm believer in putting the Atlantic between his recruits and the corrupting backgrounds from which they had been rescued," writes Harper.

Harper's academic reserve results in rather a dry tone but her documentary sources, which are in ample supply, are engrossing. The most striking impression made on the reader by Adventurers and Exiles, apart from the scale of the diaspora itself, is of the hardship of life in a cold northern country in the 19th century, especially before the perfection of the steam engine. That was helped along by a Scotsman, of course, and one who stayed at home.